Micro-Electro-Mechanical Systems, or MEMS can be defined as miniaturized devices that combine electrical and mechanical components. A microelectromechanical device typically comprises a mechanical element and an electrostatically or electromagnetically operated element, or a directly electromechanical element. MEMS devices can sense, control, and activate mechanical processes on the micro scale, and function individually or in arrays to generate effects on the macro scale.
MEMS devices can be applied to quickly and accurately detect or generate forces through very small deformations in an incorporated mass structure. For example, in electronic frequency reference applications, an electronic voltage may be used to induce a vibration in a specifically designed crystal structure. This vibration generates a corresponding output voltage with high spectral purity and stability. The mass structure exhibits resonance or resonant behavior by naturally vibrating or oscillating at some frequencies, called its resonant frequencies, with greater amplitude than at others. Mass structures deforming (vibrating, oscillating, deflecting, or otherwise exhibiting resonant behavior) in some way in a microelectromechanical device are thus herein referred to as resonators.
Conventionally, a popular technology for manufacturing frequency references has been based on quartz crystal resonators. Recent developments have, however, shown that as an enabler of component miniaturization and fabrication compatibility, silicon can eventually replace quartz in resonator structures. A primary performance characteristic of a frequency reference is stability of the generated signal. In the art, medium-term stability has been characterized to refer to changes in time-intervals of seconds to hours. Medium-term stability is dominated by temperature sensitivity, and its control is one of the most important tasks to ensure required frequency reference performance.
It has been detected that resonators with very little temperature drift, or even overcompensating resonators can be achieved by essentially homogeneously doping the deforming element with a substantial concentration on n-type doping agent. For example, document FI20115151 discloses an extensional mode beam resonator where the resonance is characterized by contraction or extension of the resonator. The document illustrates how a first order (linear) temperature coefficient of frequency (TCF) changes as the orientation of the beam resonator in respect the crystal direction changes.
It has, however, been detected that these theoretical models of prior art do not, as such, work accurately enough for practical implementations. This is especially true for resonators where the resonance is characterized by in-plane and out-of-plane flexing of the resonator. The prior art suggests to use in-plane rotation angles that zero the theoretical linear TCF curves of the first order. In the presented curves, the first order linear TCF is zeroed in approximately 21 degrees deviation from [100] crystal direction, and even higher degree optimal deviations are anticipated for high dopant concentrations. However, the frequency stability provided by the predicted optimal combinations of doping concentrations and >22 degree crystal orientations have not proved to provide adequate accuracy for industrial applications. The control of medium-term stability in flexural mode resonators seem to include various complexities than cannot be controlled by the theoretical predictions of the prior art.
One method of dealing with such complexities is suggested in a prior art document FI20115465. It proposes to manage overall temperature sensitivity properties by including in the resonator at least two types of regions having different material properties, whereby the combination of materials define an effective material. The material properties and the relative volumes are adjusted to provide desired temperature compensation characteristics such that temperature coefficients of the different regions compensate each other. It is, however, understood that without thorough understanding on monostructure behavior, it is very difficult to design and manufacture such mufti-region configurations in practice. In addition, when dealing with microscale elements, the impact of even small tolerances in the material properties, doping concentrations and element orientations are very difficult to control with required accuracy.